Add metrics event for Settings->Display->Ambient Display.

Bug: 29994493
Change-Id: Idd644854b7f63c89f4672cae68ea208d65fed0ef
diff --git a/proto/src/metrics_constants.proto b/proto/src/metrics_constants.proto
index 41576bf..fc19fb5 100644
--- a/proto/src/metrics_constants.proto
+++ b/proto/src/metrics_constants.proto
@@ -2256,6 +2256,10 @@
     // Package: Package of print service
     ACTION_PRINT_RECOMMENDED_SERVICE_INSTALL = 512;
 
+    // OPEN: Settings > Display -> Ambient Display
+    // CATEGORY: SETTINGS
+    ACTION_AMBIENT_DISPLAY = 513;
+
     // ---- End O Constants, all O constants go above this line ----
 
     // Add new aosp constants above this line.